This invention relates to high pressure unit fuel injectors wherein a fluidic timing chamber is formed between plungers of the injector for controlling the point at which the injection event occurs to thereby improve engine performance and reduce exhaust emissions. More specifically, this invention relates to injectors having a timing chamber relief valve for draining fluid from the timing chamber responsive to the fluidic pressure developed therein, for providing a sharp termination of injection and/or for obtaining increased injection pressures under slow engine speed operating conditions without exceeding the pressure capabilities of the injector at high speed operating conditions.
Commonly owned U.S. Pat. Nos. 4,721,247 to Perr and 4,986,472 to Warlick et al. describe injectors capable of operating at extremely high fuel injection pressures (on the order of 30,000 psi and above) for achieving the high levels of performance and pollution abatement demanded of modern internal combustion engines. These injectors incorporate a timing chamber formed between plungers of the injector for controlling the advance or retard of injection in relation to the pressure of a fluid, typically fuel, supplied to the timing chamber. A timing chamber relief valve is provided which serves two purposes. First, the pressure actuated valve drains timing fluid from the timing chamber, as necessary, during an injection stroke so as to achieve high injection pressures at low engine speeds while avoiding excessive injector pressures at high engine speeds. Secondly, the relief valve may function together with or in place of a spill port provided in communication with the timing chamber for collapsing the timing chamber in a controlled manner at termination of injection so as to prevent secondary injection of fuel.
The injectors of the above-mentioned patents include an injector body having a central cavity within which is received a plunger assembly comprising three plungers arranged to form the hydraulic variable timing chamber between the upper and intermediate plungers. The injection chamber is formed in the central cavity below the lower plunger.
In Perr '247, passages are provided from the timing chamber through the intermediate plunger to a valve mechanism provided between the intermediate and lower plungers. Biasing for the relief valve is provided by a single spring having the additional functions of biasing the intermediate plunger upwardly for controlling metering of fluid into the timing chamber, and controlling lifting of the lower plunger.
In Warlick et al. '472, the valve mechanism is similarly located between the lower and intermediate plungers. To improve pressure regulation using a higher spring load and to accommodate a larger area drainage passage as compared with the injector of Perr '247, a separate valve spring biases the valve mechanism toward its closed position.
As mentioned above, the injector of the Perr '247 patent uses a single spring mounted between the intermediate and lower plunger to bias the intermediate plunger upwardly. By careful design of the spring rate characteristics of the intermediate plunger biasing spring, it becomes possible to control the amount of timing fluid which is metered into the timing chamber during each cycle of injector operation by changing the pressure of the timing fluid supply to the injector. However, in the Perr '247 patent, the intermediate plunger bias spring also supplies the bias force necessary to operate the pressure actuated relief valve. Accordingly, it becomes very difficult to optimize timing fluid metering without adversely affecting the operation of the pressure actuated relief valve, and vice versa. Moreover, the size of the drain passage from the timing chamber in Perr '247 affects both the opening pressure of the pressure limiting valve and the flow rate of timing fluid drained from the timing chamber through the pressure limiting valve. These difficulties are obviated to a large extent in the injector design of Warlick et al. '472 by the provision of a relief valve spring that is separate from the timing spring, as described above.
While the injector described in the '472 patent enables the opening force of the relief valve to be adjusted without affecting the timing chamber biasing pressure, since the relief valve mechanism is still acted upon in part by the timing spring, completely independent control is not obtained. The effective biasing force acting to close the relief valve is equal to the sum of the biasing forces provided by the relief valve spring and the timing spring. Thus, since the timing spring compresses during the injection stroke, the opening force of the relief valve will vary depending upon the stroke position and movement of the lower plunger. This can make it difficult to precisely control the pressure at which the timing chamber is drained through the relief valve. Thus, there is a need for an injector having a relief valve having a bias force which is unaffected by the injection stroke of the lower plunger.
In each of the above-mentioned injectors, the timing chamber relief valve is located in a lower portion of the injector. Namely, the valve mechanism is formed between the intermediate and lower plungers and the valve biasing spring is located below the relief valve. This presents certain difficulties from a manufacturing and repair standpoint. In high pressure injection (HPI) type injectors as described above, the lower plunger is reduced substantially in diameter relative to the upper and intermediate plungers so that very high pressures in the injection chamber can be achieved without imparting such injection pressures to the timing chamber and injector drive train. More specifically, a pressure multiplication is obtained by providing the lower plunger with a pressure receiving area that is smaller than the pressure receiving areas of the upper and intermediate plungers. As a result, there is less space to accommodate the relief valve in the lower part of the plunger, i.e., below the intermediate plunger. This increases manufacturing costs and can hamper repair operations. Repair operations are further hampered by the fact that, in order to repair the relief valve, it is necessary to remove numerous injector elements, including the upper and intermediate plungers. There is, thus, a need for an injector having a timing chamber relief valve structure which facilitates assembly and repair operations.
An additional difficulty with having the relief valve mechanism formed in the lower portion of the injector is that this hampers ready adaptation of the timing chamber assembly to different types of injectors, e.g., open and closed nozzle injectors. It would be desirable if the entire timing chamber structure could be confined to an upper part of the injector such that the upper part could serve as an interchangeable injector module for use on different injectors, including both open and closed nozzle injectors.
The injectors in accordance with Perr '247 and Warlick et al. '472 have relief valve structures wherein the opening stroke of the valve seat is fixed. Thus, these references do not provide for altering the opening stroke of the relief valve in order to control more precisely the draining of fluid from the timing chamber. Moreover these references do not provide a mechanism for adjusting opening stroke independently of spring pressure.
Commonly owned U.S. Pat. No. 4,249,499 to Perr discloses a unit injector having a variable volume timing chamber formed between an upper plunger and a two-piece intermediate plunger. The intermediate plunger incorporates a pressure-sensitive relief valve for draining timing fluid from the timing chamber after the termination of injection. While the timing chamber in Perr '499 performs substantially the same function as that in Perr '247 and Warlick et al. '472, the relief valve performs only the function of controlling pressure following termination of injection. That is, the relief valve does not function to drain fluid from the timing chamber during an injection event so as to obtain an increase in injection pressures under low engine speed operating conditions without exceeding the injector pressure capability under high speed conditions.
Additionally, the relief valve means of Perr '499 is wholly contained above the lower plunger, i.e., within the two-piece intermediate plunger. In this design, the two-piece intermediate plunger adds complexity as compared with the one-piece intermediate plungers of the two previously mentioned patents and, as previously mentioned, the relief valve does not serve to control injector pressures by releasing fluid from the timing chamber during the injection event. Furthermore, since the relief valve is still below the timing chamber, difficulties are encountered in assembly and repair of the injector.
Finally, in the Perr '499 injector, the drain passages leading from the timing chamber extend to a drain conduit which is external of the engine head. Thus, the '499 patent does not teach how to utilize existing drillings in the engine head rather than an external conduit, e.g., to avoid the potential leakage to which external conduit connections are susceptible, and to avoid clutter of the engine compartment due to external fluid lines.